I am not sure if I have changed something but now I get 404 error on login. Is there anyway I can get in via PHPMyAdmin?
The Login to PHPMyAdmin and the concreteCMS dashboard are two very different things. For the login into the dashboard, you need to go through the Login controller who handles this step.
The only possible issue is that you might have renamed the Login Page or you try to get into it with a Link that does not go to it.
If you access it via address in the URL field of your browser:
yourdomain.com/index.php/login
Where does this lead you? If it leads to your Login page, the Link you clicked to access it was wrong.
If this address leads to 404 - somehow the page was renamed.
I can get to the login page but when a add the login details and hit ‘log in’ I get a 404 for /login/authenticate/concrete